This invention relates generally to methods and apparatus for CT imaging and other radiation imaging systems and, more particularly, to utilizing a generalized helical interpolation algorithm.
In at least some xe2x80x9ccomputed tomographyxe2x80x9d (CT) imaging system configurations, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as an xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through an object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at a detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged, so the angle at which the x-ray beam intersects the object constantly changes. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal spot. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator adjacent the collimator, and photodetectors adjacent to the scintillator. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield units,xe2x80x9d which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time required for multiple slices, a xe2x80x9chelicalxe2x80x9d scan may be performed. To perform a xe2x80x9chelicalxe2x80x9d scan, the patient is moved in a z-axis direction synchronously with the rotation of the gantry, while the data for a prescribed number of slices is acquired. Such a system generates a single helix from a fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.
In some CT imaging systems, the detector array is segmented so that a plurality of quasi-parallel slices of projection data are acquired and processed to construct a plurality of images corresponding to several slices through a volume. Such CT imaging systems are referred to as xe2x80x9cmultislicexe2x80x9d systems. Multislice systems provide more data compared to a single slice system for reconstructing an image. Weighting conjugate samples in multi-slice systems, similar to weighting samples in single slice systems, leads to a breakdown in image reconstruction when a large number of slices are considered.
For a single slice system, two measurements are used to interpolate/extrapolate data collected from a single rotation of the gantry onto a xe2x80x9cplane of reconstructionxe2x80x9d (POR). Measurements acquired at different source positions are known as xe2x80x9cconjugate measurements.xe2x80x9d The conjugate measurements are used to generate a conjugate sample line. Multislice CT imaging systems increase the flexibility in choosing a row or a plurality of rows that provide data closest to the POR. This flexibility of choosing a row may exist at both ends of a given ray, i.e., for both conjugate measurements for the same ray through the patient. Pitches for which conjugate measurements are available are known as HQ pitches.
The POR line intersects the conjugate sample line at an intersection point. At the intersection point, a weight becomes in-deterministic because the weight at the POR line should be unity and the weight for the conjugate sample line should be zero. Known algorithms dynamically switch the interpolation pairs so that two samples that are located closest to the POR are used for interpolation to avoid in-determinancy. At the point of switching, however, discontinuity in the weights occur. This discontinuity, after filter and backprojection, leads to a periodic rippling pattern from the center region of the reconstructed image to the outer edge of field of view. Typically, as a method of compensation, a feathering technique is used to bridge gaps in the weights. Yet, image artifacts can not be avoided, since the filtering operation in the image reconstruction is essentially a derivative operator that enhances any small discrepancies in the interpolated samples.
Methods and apparatus for a multislice computed tomographic (CT) imaging system to reduce image artifacts when reconstructing an image are provided. In an exemplary embodiment of the method, an object is helically scanned to obtain projection data for a plurality of projection views of the object, a plane of reconstruction is defined, a conjugate sample line is defined that does not intersect the plane of reconstruction, a weighting function is applied to the conjugate samples to reduce image artifacts, and the image is reconstructed after the weighted data is filtered and backprojected.
In another aspect, an imaging system includes a computer, a gantry having a detector array, an x-ray source for radiating an x-ray beam toward the detector array, and the imaging system acquires projection data for a plurality of projection views of the object, defines a plane of reconstruction, defines a conjugate sample line that does not intersect the plane of reconstruction, applies a weighting function to the conjugate samples to reduce image artifacts, and reconstructs the image after the weighted data is filtered and backprojected.
In another aspect, a processor in the imaging system is programmed to acquire projection data for a plurality of projection views of the object, define a plane of reconstruction, define a conjugate sample line such that it does not intersect the plane of reconstruction, apply a weighting function to the conjugate samples to reduce image artifacts, and reconstruct the image after the weighted data is filtered and backprojected.
In yet another aspect, a computer-readable medium in the imaging system is provided which comprises a plurality of records of projection data used to define a plane of reconstruction. In addition, a record of conjugate samples is stored on the computer-readable medium. A program residing on the computer-readable medium utilizes a plurality of rules to define a conjugate sample line based on the record of conjugate samples which does not intersect the plane of reconstruction. Further, the program includes a plurality of rules to determine a weighting function that is applied to the conjugate samples to reduce image artifacts and reconstruct the image.